Method for producing carbon molecular sieve membranes in controlled atmospheres

ABSTRACT

The invention concerns carbon molecular sieve membranes (“CMS membranes”), and more particularly the use of such membranes in gas separation. In particular, the present disclosure concerns an advantageous method for producing CMS membranes with desired selectivity and permeability properties. By controlling and selecting the oxygen concentration in the pyrolysis atmosphere used to produce CMS membranes, membrane selectivity and permeability can be adjusted. Additionally, oxygen concentration can be used in conjunction with pyrolysis temperature to further produce tuned or optimized CMS membranes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to provisional application No.61/256,097 filed Oct. 29, 2009, the entire content of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

The invention concerns carbon molecular sieve membranes (“CMSmembranes”), and more particularly the use of such membranes in gasseparation. In particular, the present disclosure concerns anadvantageous method for producing CMS membranes with desired selectivityand permeability properties. It has been discovered that by controllingand selecting the oxygen concentration in the pyrolysis atmosphere usedto produce CMS membranes, membrane selectivity and permeability can beadjusted. In particular, embodiments of the invention include optimizingacid gas permeability or selectivity by varying the oxygen concentrationin the pyrolysis atmosphere. Further embodiments of the inventioninclude using a combination of oxygen concentration and pyrolysistemperature to tune CMS performance.

Membranes are widely used for the separation of gases and liquids,including for example, separating acid gases, such as CO₂ and H₂S fromnatural gas, and the removal of O₂ from air. Gas transport through suchmembranes is commonly modeled by the sorption-diffusion mechanism.Specifically, gas molecules sorb into the membrane at the upstream,diffuse through the membrane, and finally desorb from the membrane atthe downstream. Two intrinsic properties are commonly used to evaluatethe performance of a membrane material; “permeability” and“selectivity.” Permeability is hereby defined as a measure of theintrinsic productivity of a membrane material; more specifically, it isthe partial pressure and thickness normalized flux, typically measuredin Barrer. Selectivity, meanwhile, is a measure of the ability of onegas to permeate through the membrane versus a different gas; forexample, the permeability of CO₂ versus CH₄, measured as a unit-lessratio.

Currently, polymeric membranes are well studied and widely available forgaseous separations due to easy processability and low cost. CMSmembranes, however, have been shown to have attractive separationperformance properties exceeding that of polymeric membranes.

CMS membranes are typically produced through thermal pyrolysis ofpolymer precursors. For example, it is known that defect-free hollowfiber CMS membranes can be produced by pyrolyzing cellulose hollowfibers (J. E. Koresh and A. Soffer, Molecular sieve permselectivemembrane. Part I. Presentation of a new device for gas mixtureseparation. Separation Science and Technology, 18, 8 (1983)). Inaddition, many other polymers have been used to produce CMS membranes infiber and dense film form, among which polyimides have been favored.Polyimides have a high glass transition temperature, are easy toprocess, and have one of the highest separation performance propertiesamong other polymeric membranes, even prior to pyrolysis.

U.S. Pat. No. 6,565,631 to Koros et al., which is incorporated herein byreference, describes a method of synthesizing CMS membranes. Inparticular, a polyimide hollow fiber was placed in a pyrolysis furnacewith an evacuated environment, with a pyrolysis pressure of between 0.01and 0.10 mm Hg air. U.S. Pat. No. 6,565,631 also discloses a method ofusing CMS membranes to separate CO₂ from a methane stream containing 10%CO₂, at 1000 psia and 50° C., with a selectivity of approximately 45, aselectivity that is much higher than typical commercial polymericmembranes. U.S. Pat. No. 6,565,631 also discloses that CMS membranescan, unlike polymeric membranes, operate with trace amounts ofhydrocarbon impurities with little loss in selectivity. Other patentsthat describe processes for producing carbon membranes (both asymmetrichollow “filamentary” and flat sheets), and applications for gasseparation, include U.S. Pat. No. 5,288,304, and EP Patent No. 459,623,which are incorporated herein in their entireties.

Prior research has shown that CMS membrane separation properties areprimarily affected by the following factors: (1) pyrolysis precursor,(2) pyrolysis temperature, (3) thermal soak time, and (4) pyrolysisatmosphere. The first three factors have been investigated in detail,but the effect of the fourth factor, pyrolysis atmosphere, has remainedlargely unknown.

For example, Steel and Koros performed a detailed investigation of theimpact of pyrolysis temperature, thermal soak time, and polymercomposition on the performance of carbon membranes. (K. M. Steel and W.J. Koros, Investigation of Porosity of Carbon Materials and RelatedEffects on Gas Separation Properties, Carbon, 41, 253 (2003).) Membraneswere produced in an air atmosphere at 0.05 mm Hg pressure. The resultsshowed that increases in both temperature and thermal soak timeincreased the selectivity but decreased permeance for CO₂/CH₄separation. In addition, Steel et al showed that a precursor polymerwith a rigid, tightly packed structure tends to lead to a CMS membranehaving higher selectivity compared with less rigid precursor polymers.

The impact of pyrolysis atmosphere has been researched only to a limitedextent. Suda and Haraya disclosed the formation of CMS membranes underdifferent environments. (H. Suda and K. Haraya, Gas Permeation ThroughMicropores of Carbon Molecular Sieve Membranes Derived From KaptonPolyimide, J. Phys. Chem. B, 101, 3988 (1997).) CMS dense films wereprepared from polyimide Kapton® at 1000° C. in either argon or invacuum. According to their gas separation properties, the results of anO₂/N₂ separation were almost the same between 6 membranes formed underthe different atmospheres. Suda and Haraya did not disclose the effectsof atmosphere on CO₂ separation from natural gas, nor disclose howseparation properties vary with oxygen concentration. Similarly,Geiszler and Koros disclosed the results of CMS fibers produced frompyrolysis of fluorinated polyimide in helium and argon for both O₂/N₂and H₂/N₂ separations. (V. C. Geiszler and W. J. Koros, Effects ofPolyimide Pyrolysis Atmosphere on Separation Performance of CarbonMolecular Sieve Membranes, J. Memb. Sci., (2009).). That paper discloseda slightly higher selectivity and lower permeability with vacuumpyrolysis than the purged pyrolysis processes. In addition, Geiszler andKoros showed that the flow rate of the purge gases impacted performance.Geiszler and Koros, however, did not disclose the effects of atmosphereon CO₂ separation from natural gas, or the effects of oxygenconcentration on separation properties.

The present inventors recently extended the study of pyrolysisenvironments, and proposed in publications that a critical factorimpacting the separation performance of CMS membranes is oxygen exposureduring pyrolysis. (M. Kiyono, P. J. Williams, and W. J. Koros, Effect ofPyrolysis Atmosphere on Separation Performance of Carbon Molecular SieveMembranes, J. Memb. Sci., (2009); P. J. Williams, Analysis of FactorsInfluencing the Performance of CMS Membrane for Gas Separation, GeorgiaInstitute of Technology (2006).) In the paper by Kiyono et al., thepyrolysis environment was purged with specialty gases containingcontrolled amounts of oxygen ranging from 4-50 ppm O₂ in argon flowingat 200 cc(STP)/min. The performance of the membranes was found to be astrong function of oxygen exposure. Neither paper, however, disclosedthe effect of oxygen concentration, rather the total oxygen amount, onseparation properties. The present inventors have since discovered thatoxygen concentration, and not total oxygen amount, impacts the overallseparation performance of the membranes and allows the membraneperformance to be modified to optimize selectivity and permeability.

SUMMARY OF THE INVENTION

An aspect of the invention concerns a process for making a carbonmembrane including providing a polymer precursor, heating the precursorin a chamber to at least a temperature at which pyrolysis byproducts areevolved, and flowing an inert gas through the chamber, the inert gascontaining less than about 40 ppm of oxygen.

Another aspect of the invention concerns a process for reducing theconcentration of acid gases in a natural gas stream that includesproviding a carbon membrane produced by a process including the steps ofproviding a membrane formed of asymmetric hollow polymer fibers, heatingthe membrane in a chamber to at least a temperature at which pyrolysisbyproducts are evolved, and flowing an inert gas through the chamber,the inert gas containing less than about 40 ppm of oxygen.

Another aspect of the invention is a process for optimizing the CO₂/CH₄selectivity of a carbon membrane, the process including forming themembrane by providing a polymer precursor in the form of asymmetrichollow polymer fibers, heating the precursor in a chamber to at least atemperature at which pyrolysis byproducts are evolved, and flowing aninert gas through the chamber, the inert gas containing less than about40 ppm of oxygen.

A further aspect of the invention is a process of reducing theconcentration of acid gas in a natural gas feed stream comprisingmethane, acid gas (such as CO₂ or H₂S), and other natural gascontaminants such as heavy hydrocarbons, the process comprisingdirecting the feed stream through a membrane produced by the process ofproviding a polymer precursor, heating the precursor in a chamber to atleast a temperature at which pyrolysis byproducts are evolved, andflowing an inert gas through the chamber during the heating step, theinert gas containing less than about 40 ppm of oxygen, to produce aretentate gas stream having a reduced concentration of the acid gasrelative to the feed stream and a permeate gas stream having anincreased concentration of the acid gas relative to the feed stream.

Another aspect of the invention is a process for making a carbonmembrane having a predetermined degree of CO₂ permeability, the processincluding providing a polymer precursor, heating the precursor in achamber to at least a temperature at which pyrolysis byproducts areevolved, and flowing a gas through the chamber during the heating step,the concentration of oxygen in the gas being selected to produce acarbon membrane having the predetermined degree of CO₂ permeability.

Yet another aspect of the invention is a process for making a carbonmembrane having a predetermined degree of CO₂/CH₄ selectivity, theprocess including providing a polymer precursor, heating the precursorin a chamber to at least a temperature at which pyrolysis byproducts areevolved, and flowing a gas through the chamber, the concentration ofoxygen in the gas being selected to produce a carbon membrane having thepredetermined degree of CO₂/CH₄ selectivity.

Another aspect of the invention concerns a gas separation apparatusincluding at least two carbon membranes having different gas separationproperties. At least one membrane is produced by pyrolyzing a polymerprecursor in an atmosphere having a first predetermined oxygenconcentration, and at least another membrane is produced by pyrolyzing apolymer precursor in an atmosphere having a different predeterminedoxygen concentration, the respective oxygen concentrations differing byat least 2 ppm oxygen to inert gas. Optionally, the apparatus mayinclude at least two carbon membranes produced by separately pyrolyzingpolymer precursors in atmospheres having oxygen concentrations differingby at least 4 ppm oxygen to inert gas, alternatively 6 ppm oxygen toinert gas, and alternatively 10 ppm oxygen to inert gas. For example,one carbon membrane may be provided which has a very high permeabilitybut lower selectivity, while a second carbon membrane may be providedwhich has a lower permeability and higher selectivity.

A still further aspect of the invention concerns a process for makingtwo or more carbon membranes having different predetermined degrees ofCO₂ permeability, the process including providing a first polymerprecursor, heating the first precursor in a first chamber to at least atemperature at which pyrolysis byproducts are evolved, flowing a firstgas through the first chamber during the heating step, the concentrationof oxygen in the first gas being selected to produce a carbon membranehaving a first predetermined degree of CO₂ permeability. Then, theprocess includes providing a second polymer precursor, heating thesecond precursor in a second chamber to at least a temperature at whichpyrolysis byproducts are evolved, and flowing a second gas through thesecond chamber during the heating step, the concentration of oxygen inthe second gas being selected to produce a carbon membrane having asecond predetermined degree of CO₂ permeability. The concentration ofoxygen in the first gas differs from that of the second gas in thisembodiment by at least 2 ppm, alternatively at least 4 ppm,alternatively at least 6 ppm, alternatively at least 10 ppm,alternatively at least 15 ppm.

Similarly, another aspect of the invention concerns a process for makingtwo or more carbon membranes having different predetermined degrees ofCO₂/CH₄ selectivity. The process includes providing a first polymerprecursor, heating the first precursor in a first chamber to at least atemperature at which pyrolysis byproducts are evolved, flowing a firstgas through the first chamber during the heating step, the concentrationof oxygen in the first gas being selected to produce a carbon membranehaving a first predetermined degree of CO₂/CH₄ selectivity. Then, theprocess comprises providing a second polymer precursor, heating thesecond precursor in a second chamber to at least a temperature at whichpyrolysis byproducts are evolved, and flowing a second gas through thesecond chamber during the heating step, the concentration of oxygen inthe second gas being selected to produce a carbon membrane having asecond predetermined degree of CO₂/CH₄ selectivity. The concentration ofoxygen in the first gas differs from that of the second gas in thisembodiment by at least 2 ppm, alternatively at least 4 ppm,alternatively at least 6 ppm, alternatively at least 10 ppm,alternatively at least 15 ppm.

Other aspects of the invention will be apparent from this disclosure andthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary pyrolysis apparatus for synthesis of carbonmolecular sieve membrane films.

FIG. 2 is a schematic of the oxygen doping process during pyrolysis.

FIG. 3 is a chart of separation performance of 6FDA/BPDA-DAM carbonmolecular sieve dense films, showing CO₂ permeability and CO₂/CH₄selectivity as a function of varying oxygen concentration in thepyrolysis atmosphere. Permeability and selectivity of the non-pyrolyzed6FDA/BPDA-DAM precursor polymer is also shown.

FIG. 4 is a chart of separation performance of Matrimid® derived carbonmolecular sieve membranes, showing CO₂ permeability and CO₂/CH₄selectivity as a function of varying oxygen concentration in thepyrolysis atmosphere. Permeability and selectivity of the non-pyrolyzedMatrimid® precursor polymer is also shown.

FIG. 5 is a chart of separation performance of CMS membranes producedfrom Matrimid® polymeric precursor, showing CO₂ permeability and CO₂/CH₄selectivity as a function of varying soak time.

FIG. 6 is a chart of separation performance of CMS membranes producedfrom Matrimid® polymeric precursor, showing CO₂ permeability and CO₂/CH₄selectivity as a function of varying pyrolysis atmosphere flow rate.

FIG. 7 is a chart of separation performance of CMS membranes producedfrom Matrimid® polymeric precursor, showing CO₂ permeability and CO₂/CH₄selectivity as a function of varying precursor polymer thickness.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which one or more embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments described herein. Rather, these embodiments are examples ofthe invention, which has the full scope indicated by the language of theclaims. Like numbers refer to like elements throughout.

In the following examples and embodiments, methods for producing CMSmembranes are provided. The CMS membranes can advantageously haveoptimized gas separation performance properties, such properties beingoptimized by controlling the concentration of oxygen in the pyrolysisatmosphere.

Polymeric Precursor Fibers or Films

A polymeric material is the starting material for preparation of thepresent carbon molecular sieve membranes. The polymeric material isalternatively a polymeric fiber or a polymeric film.

One useful polymer precursor is Matrimid® 5218, a commercially availablepolyimide available from Huntsman Advanced Materials (formerly Vantico,Inc.). Matrimid® 5218 is a thermoplastic polyimide based on aproprietary diamine, 5(6)-amino-1-(4′ aminophenyl)-1,3,-trimethylindane.An alternative polymer precursor is 6FDA/BPDA-DAM, a polyimidesynthesized by the thermal imidization method from three monomers:2,4,6-trimethyl-1,3-phenylene diamine,5,5′-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]-1,3-isobenzofurandion,and 3,3′,4,4′-biphenyl tetra carboxylic acid dianhydride, all availablefrom Sigma Aldrich, St. Louis, Mo. The chemical structure of6FDA/BPDA-DAM is shown below:

A 1:1 ratio of components X and Y may advantageously be used.

Both Matrimid® 5218 and 6FDA/BPDA-DAM are advantageously initiallyprovided as polymeric powders. In one embodiment, homogenous polymericdense films are prepared from the polymeric powder by any suitablemeans. For example, the polymeric powders can be dried in a vacuum ovento remove moisture, dissolved in a suitable solvent, and prepared intopolymer dense films by the solution casting method. After solutioncasting, the films may be again dried in a vacuum oven to removeresidual solvent. Once the films are prepared, they may be cut intosmall discs suitable for use in a permeation cell.

In another embodiment, as described in U.S. Pat. No. 6,565,631, thepolymer precursors may be provided as polymeric fibers. The polymericfiber, advantageously Matrimid® or 6FDA/BPDA-DAM, may be spun by anyconventional method, e.g., spun from a polymer solution through aspinneret. Such fibers are available from E.I. du Pont de Nemours andCompany and L′Air Liquide S. A. For example, such a polymer is describedin U.S. Pat. No. 5,234,471, which disclosure is incorporated byreference in its entirety. Such fibers may be symmetric (i.e., have aconsistent morphology) or asymmetric (i.e., having two or morestructural planes of non-identical morphologies). Without limiting thepresent disclosure, commercially available polymeric fibers areasymmetric and typically have an outer diameter of about 250 μm and aninner diameter of about 160 μm.

Pyrolysis of Polymeric Precursor to Create CMS Membranes

Polymeric films or fibers may then be pyrolyzed to produce CMSmembranes. In the case of polymeric films, the films may be placed on aquartz plate, which is optionally ridged to allow for the diffusion ofvolatile by-products from the top and bottom of the films into theeffluent stream. The quartz plate and films may then be loaded into apyrolysis setup. In the case of polymeric fibers, the fibers may beplaced on the quartz plate and/or a piece of stainless steel mesh andheld in place by any conventional means, e.g., by wrapping a length ofbus wire around the mesh and fibers. The mesh support and fibers maythen be loaded into the pyrolysis setup. In another embodiment, thefibers may be secured on one of both ends by any suitable means andplaced vertically in a pyrolysis chamber. Additional methods may also beused to place the polymer fibers into a pyrolysis setup.

1. Pyrolysis Equipment

FIG. 1 illustrates an exemplary pyrolysis setup. Other suitablepyrolysis equipment, however, as known in the art may be used, and FIG.1 is not intended to limit the present invention. As shown in FIG. 1, atemperature controller 101 is used to heat a furnace 100 and a quartztube 102. An assembly 103 of a metal flange with silicon O-rings may beused on both ends of the quartz tube to seal the tube to reduce leakswhen performing pyrolysis under vacuum. For vacuum pyrolysis, a pump(not shown) is provided (for example, an Edwards model RV3) that iscapable of creating a low pressure from 0.005 to 0.042 torr, and aliquid nitrogen trap (not shown) may be used to prevent any backdiffusion of oil vapor from the pump. The pressure inside the tube maybe monitored with a pressure transducer 105 (for example, an MKSInstruments 628B capacitance manometer with 0.5% accuracy below 1 torr)attached to a digital read-out 106 (for example, an MKS InstrumentsPDR2000). For processes using purged gas during pyrolysis, an inert gassource 107 is provided, with a micro needle valve 108 installed in thegas line for permitting a flow of oxygen (air) into the purge gas. Theflow rate of the gas may be controlled with a mass flow controller 109(for example, MKS Instruments type 247), and confirmed with a bubbleflow meter (not shown, for example, Fisher Scientific model 520) beforeand after each process. Any oxygen analyzer 104, for example a CambridgeSensotec Ltd. Rapidox 2100 series with ±1% accuracy between 10⁻²⁰ ppmand 100% may be integrated with the system to monitor oxygenconcentration during the pyrolysis process. Between processes, thequartz tube and plate are optionally rinsed with acetone and baked inair at 800° C. to remove any deposited materials which could affectconsecutive runs.

2. Pyrolysis Heating Parameters

U.S. Pat. No. 6,565,631 describes a heating method for pyrolysis ofpolymeric fibers to form CMS membranes, and is incorporated herein byreference. For either polymeric films or fibers, a pyrolysis temperatureof between about 450° C. to about 800° C. may advantageously be used,although as discussed below, the pyrolysis temperature can be adjustedin combination with the pyrolysis atmosphere to tune the performanceproperties of the resulting CMS membrane. For example, the pyrolysistemperature may be 1000° C. or more. Optionally, the pyrolysistemperature is maintained between about 500° C. and about 550° C. Thepyrolysis soak time (i.e., the duration of time at the pyrolysistemperature) may vary (and may include no soak time) but advantageouslyis between about 1 hour to about 10 hours, alternatively from about 2hours to about 8 hours, alternatively from about 4 hours to about 6hours. An exemplary heating protocol may include starting at a first setpoint of about 50° C., then heating to a second set point of about 250°C. at a rate of about 13.3° C. per minute, then heating to a third setpoint of about 535° C. at a rate of about 3.85° C. per minute, and thena fourth set point of about 550° C. at a rate of about 0.25° C. perminute. The fourth set point is then optionally maintained for thedetermined soak time. After the heating cycle is complete, the system istypically allowed to cool while still under vacuum or in a controlledatmosphere.

3. Pyrolysis Atmosphere

Embodiments of the present disclosure advantageously utilize acontrolled purge gas atmosphere during pyrolysis. It has been found thatby varying the concentration of oxygen in the pyrolysis atmosphere, onecan control or tune the gas separation performance properties of theresulting CMS membrane. By way of example, an inert gas such as argon isused as the purge gas atmosphere. Other suitable inert gases include,but are not limited to, nitrogen, helium, or any combinations thereof.By using any suitable method such as a valve, the inert gas containing aspecific concentration of oxygen may be introduced into the pyrolysisatmosphere. For example, the amount of oxygen in the purge atmosphere isless than about 50 ppm (parts per million) O₂/Ar. Alternatively, theamount of oxygen in the purge atmosphere is less than 40 ppm O₂/Ar.Embodiments of the present disclosure may also use pyrolysis atmosphereswith about 8 ppm, 7 ppm, or 4 ppm O₂/Ar. As discussed in more detailbelow, by including a small amount of oxygen in the pyrolysisatmosphere, one can dope the CMS membrane material with oxygen in acontrolled manner, to achieve predetermined gas separation performance.

Alternatively, pyrolysis may be performed under vacuum. If a vacuum isused, the pressure during pyrolysis is advantageously from about 0.01 mmHg to about 0.10 mm Hg. In one alternative embodiment, the system isevacuated until the pressure is 0.05 mm Hg or lower.

Construction of CMS Membrane Permeation Cells

Once CMS membranes are prepared, they may be loaded or assembled intosuitable permeation cells or modules. For example, if a CMS film isused, it may be first masked using impermeable aluminum tape, and only aspecific area exposed for permeation. Epoxy (e.g., Devcon, Danvers,Mass.) may be applied at the interface of the tape and the film tofurther minimize any gas leak. Such an assembly may then optionally beplaced in a permeation cell, such as a double O-ring flange permeationcell.

If CMS fibers are used, a suitable plurality of the pyrolyzed fibers maybe bundled together to form a separation unit. The number of fibersbundled together will depend on fiber diameters, lengths, and on desiredthroughput, equipment costs, and other engineering considerationsunderstood by those in the chemical engineering arts. The fibers may beheld together by any conventional means. This assembly may thentypically be disposed in a pressure shell such that one end of the fiberassembly extends to one end of the pressure shell and the opposite endof the fiber assembly extends to the opposite end of the pressure shell.The fiber assembly is then fixably or removably affixed to the pressureshell by any conventional method to form a pressure tight seal.

For industrial use, a permeation cell or module made using eitherpyrolyzed film or fibers may be operated, as described in U.S. Pat. No.6,565,631, e.g., as a shell-tube heat exchanger, where the feed ispassed to either the shell or tube side at one end of the assembly andthe product is removed from the other end. For maximizing high pressureperformance, the feed is advantageously fed to the shell side of theassembly at a pressure of greater than about 10 barr, and alternativelyat a pressure of greater than about 40 barr. The feed may be any gashaving a component to be separated, such as a natural gas feedcontaining an acid gas such as CO₂. For example, the feed gas maycontain at least about 1% acid gas, or alternatively at least about 3%acid gas. At least a portion of the acid gas in the feed mayadvantageously be passed through the membrane to the tube side, i.e.,inside the membranes. Acid-gas-depleted feed is then removed from theopposite end of the shell side of the assembly. Any conventional recyclescheme may be used to optimize a desired purity level.

In order to perform small-scale permeation tests, a test module may beconstructed. When using CMS films, the permeation cell may be placed ina permeation system in which a constant-volume variable-pressure methodis utilized. Exemplary methods of constructing such a permeation systemhave been disclosed by Pye, et al. (D. G. Pye, H. H. Hoehn, and M.Panar, Measurement of Gas Permeability of Polymers I, J. Appl. Polym.Sci., 20, 1921 (1976) and D. G. Pye, H. H. Hoehn, and M. Panar,Measurement of Gas Permeability of Polymers II, J. Appl. Polym. Sci.,20, 287 (1976).) Both upstream and downstream of the permeation systemare evacuated for at least 12 hours, and a leak rate for the entirepermeation system is measured, which is preferably less than 1% of thepermeability of the slowest gas. Once the system is evacuated, theupstream is pressured with a testing gas while the downstream ismaintained at a vacuum, but isolated from the vacuum pump. The pressurerise in a standard volume on the downstream can be calculated with timeby a data acquisition software, such as LabView (National Instruments,Austin, Tex.) and permeability can be calculated. The system mayadvantageously be evacuated each time before experiments with differentgases for at least 12 hours.

To perform small-scale permeation tests using CMS fibers, a test moduleconsisting of a single CMS fiber may be constructed and tested asdescribed in U.S. Pat. No. 6,565,631.

Tuning Gas Separation Performance Properties of CMS Membranes

The described preparation of CMS membranes leads to an almost purecarbon material. Such materials are believed to have a highly aromaticstructure comprising disordered sp² hybridized carbon sheet, a so-called“turbostratic” structure. The structure can be envisioned to compriseroughly parallel layers of condensed hexagonal rings with no long rangethree-dimensional crystalline order. Pores are formed from packingimperfections between microcrystalline regions in the material, and porestructure in CMS membranes is known to have a slit-like structure. Thestructure has bimodal pore size distribution of micropore andultramicropore, which is known to be responsible for the molecularsieving gas separation process.

The micropores are believed to provide adsorption sites, andultramicropores are believed to act as molecular sieve sites. Theultramicropores are believed to be created at “kinks” in the carbonsheet, or from the edge of a carbon sheet. These sites have morereactive unpaired sigma electrons prone to oxidation than other sites inthe membrane. Based on this fact, it is believed that by tuning theamount of oxygen exposure, the size of selective pore windows can betuned. It is also believed that tuning oxygen exposure results in oxygenchemisorption process on the edge of the selective pore windows

Specifically, it has been found that for pyrolyzed polyimides inparticular, gas separation performance can be tuned by “doping” the CMSmembrane with oxygen in a controlled manner, as shown in FIG. 2. At agiven pyrolysis protocol (temperature, ramp rate, soak time), anintrinsic carbon structure is formed due to the decomposition of thepolymer followed by some compaction of the resulting amorphous carbon.When oxygen is added, another process occurs whereby the oxygen isincorporated into the intrinsic carbon structure and changes the poresize distribution. Selective pore sizes in the range of 3.4 to 4.2angstroms provide high CO₂/CH₄ selectivity. Pores of about 3.8 angstromshave the advantage of also providing high permeability.

In CMS membranes made from 6FDA/BPDA-DAM polyimide, a pyrolysis oxygenconcentration of less than about 40 ppm O₂/inert gas provides optimizedCO₂/CH₄ selectivity. Optionally, an oxygen concentration of between 8ppm O₂/inert gas and 40 ppm O₂/inert gas is believed to provide thehighest CO₂/CH₄ selectivity, particularly for membranes produced attemperatures below 550° C.

For CMS membranes made from Matrimid® 5218 polyimide based on a diamine,5(6)-amino-1-(4′ aminophenyl)-1,3,-trimethylindane, both CO₂/CH₄selectivity and CO₂ permeability decrease with increasing pyrolysisatmosphere oxygen concentration. If high CO₂/CH₄ selectivity and highCO₂ permeability is desired, one may pyrolyze the polymer in anatmosphere having an oxygen concentration of less than about 10 ppmO₂/inert gas.

The present methods can further be utilized by tuning pyrolysistemperature in conjunction with the oxygen concentration in thepyrolysis atmosphere. As disclosed by Steel, et al., which isincorporated by reference herein, higher pyrolysis temperature leads tolower permeability and higher selectivity. It is believed that loweringpyrolysis temperature produces more open CMS structures. This can,therefore, make the doping process more effective in terms of increasingselectivity for challenging gas separations for intrinsically permeablepolymer precursors. Therefore, by controlling the pyrolysis temperatureand the concentration of oxygen one can tune oxygen doping and,therefore, gas separation performance. In general, more oxygen andhigher temperature leads to smaller pores. Higher temperatures generallycause the formation of smaller micro and ultramicropores, while moreoxygen generally causes the formation of small selective ultramicroporeswithout having a significant impact on the larger micropores into whichgases are absorbed.

The combination of oxygen concentration and pyrolysis temperature,therefore, provides enhanced tuning of CMS performance. For example, inCMS membranes made from 6FDA/BPDA-DAM polyimide, if high CO₂/CH₄selectivity is desired, one may advantageously use a pyrolysis oxygenconcentration of between about 8 ppm O₂/inert gas and about 40 ppmO₂/inert gas, together with a pyrolysis temperature of greater than 550°C. and optionally up to about 1000° C. If high CO₂ permeability isdesired, one may advantageously use a pyrolysis oxygen concentration ofless than about 30 ppm O₂/inert gas, and alternatively a pyrolysisoxygen concentration of less than about 8 ppm O₂/inert gas, togetherwith a pyrolysis temperature of less than about 550° C. andalternatively less than about 500° C.

Similarly, in CMS membranes made from Matrimid® 5218 polyimide based ona diamine, 5(6)-amino-1-(4′ aminophenyl)-1,3, -trimethylindane, if highCO₂/CH₄ selectivity is desired, it is advantageous to use a pyrolysisoxygen concentration of less than about 40 ppm O₂/inert gas,alternatively less than about 8 ppm O₂/inert gas, together with apyrolysis temperature of greater than about 550° C. and alternatively upto about 1000° C. If a lower CO₂/CH₄ selectivity is desired, it isadvantageous to use a pyrolysis oxygen concentration of greater thanabout 40 ppm O₂/inert gas or less than about 8 ppm O₂/inert gas,alternatively greater than about 50 ppm O₂/inert gas, together with apyrolysis temperature of greater than 550° C., optionally up to about1000° C.

One embodiment of the present invention, therefore, is a gas separationapparatus having at least two carbon membranes having different gasseparation properties. At least one membrane is produced by pyrolyzing apolymer precursor in an atmosphere having a first predetermined oxygenconcentration, and at least another membrane is produced by pyrolyzing apolymer precursor in an atmosphere having a different predeterminedoxygen concentration, the respective oxygen concentrations differing byat least 2 ppm oxygen to inert gas. Optionally, the apparatus mayinclude at least two carbon membranes produced by separately pyrolyzingpolymer precursors in atmospheres having oxygen concentrations differingby at least about 4 ppm oxygen to inert gas, alternatively about 6 ppmoxygen to inert gas, alternatively about 10 ppm oxygen to inert gas,alternatively about 15 ppm oxygen to inert gas. For example, one carbonmembrane may be provided which has a very high permeability but lowerselectivity, while a second carbon membrane may be provided which has alower permeability and higher selectivity. Alternatively, two or morecarbon membranes may have, for example, a CO₂/CH₄ selectivity differingone from the other by about 10 or more, and alternatively by about 20 ormore, alternatively by about 30 or more, alternatively by about 50 ormore. Optionally, two or more carbon membranes may have, for example,CO₂ permeabilities differing one from the other by at least about 10Barrer, alternatively about 10 Barrer, alternatively about 50 Barrer,alternatively about 100 Barrer, alternatively about 200 Barrer. This isparticularly useful in an embodiment in which one or more membranes areused in series, wherein the concentration of, for example, CO₂ or H₂S ina feed gas stream is depleted as the gas stream passes from one membraneto another.

Another embodiment of the present invention is a process for making twoor more carbon membranes having different predetermined degrees of CO₂permeability or CO₂/CH₄ selectivity. A first polymer precursor and asecond polymer precursor are provided. The first precursor is heated ina first chamber to at least a temperature at which pyrolysis byproductsare evolved, for example about 500° C. to about 550° C., alternativelygreater than 600° C., alternatively greater than 700° C., alternativelygreater than 800° C. A first gas is flowed through the first chamberduring the heating step, the concentration of oxygen in the first gasselected to produce a carbon membrane having a first predetermineddegree of CO₂ permeability. The concentration of oxygen in the first gasmay be, for example, between 2 ppm O₂/inert gas to 40 ppm O₂/inert gas,alternatively 4 ppm O₂/inert gas to 30 ppm O₂/inert gas, alternatively 4ppm O₂/inert gas to 10 ppm O₂/inert gas. The second precursor is heatedin a second chamber to at least a temperature at which pyrolysisbyproducts are evolved. The temperature may be the same as the heatingtemperature for the first polymer precursor, or may differ by about 50°C., alternatively by about 100° C., alternatively by about 200° C.,alternatively by about 300° C. A second gas is flowed through the secondchamber during the heating step, the concentration of oxygen in thesecond gas being selected to produce a carbon membrane having a secondpredetermined degree of CO₂ permeability. The concentration of oxygen inthe second gas differs from the concentration in the first gas by about2 ppm oxygen, alternatively by about 4 ppm oxygen, alternatively byabout 6 ppm oxygen, alternatively by about 10 ppm oxygen, alternativelyby about 20 ppm oxygen, alternatively by about 25 ppm oxygen,alternatively by about 30 ppm oxygen. For example, one carbon membranemay be provided which has a very high permeability but lowerselectivity, while a second carbon membrane may be provided which has alower permeability and higher selectivity. Alternatively, two or morecarbon membranes have, for example, a CO₂/CH₄ selectivity differing onefrom the other by about 10 or more, alternatively, by at least about 20or more, alternatively by at least about 30 or more, alternatively by atleast about 50 or more. Optionally, two or more carbon membranes have,for example, CO₂ permeabilities differing one from the other by at leastabout 10 Barrer, alternatively by at least about 10 Barrer,alternatively by at least about 50 Barrer, alternatively by at leastabout 100 Barrer, alternatively by at least about 200 Barrer.

EXAMPLES

The following examples illustrate several of the exemplary embodimentsof the present disclosure. The examples relate to two useful polymericprecursors, and they describe results for specific pyrolysistemperature, oxygen concentrations, and the like. One of ordinary skillin the art will appreciate, however, based on the foregoing detaileddescription, how to conduct the following exemplary methods using othersuitable polymeric precursors, and with varying pyrolysis parameters,and how to scale the examples to industrial applications.

Example 1 CMS Films from 6FDA/BPDA-DAM Polyimide

A CMS film is prepared using a 6FDA/BPDA-DAM polyimide synthesized bycondensation followed by the thermal imidization from three monomers:2,4,6-trimethyl-1,3-phenylene diamine (DAM),5,5-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]-1,3-isobenzofurandion(6FDA), and 3,3′,4,4′-biphenyl tetra carboxylic acid dianhydride (BPDA),all available from Sigma Aldrich, St. Louis, Mo. In this study, thereaction stoichiometry was adjusted to have the ratio of BPDA to DAM of1:1. Homogenous polymeric dense films were prepared by first drying thepolymer powder in a vacuum oven at 110° C. for at least 12 hours toremove moisture. Then, the powder was dissolved in dichloromethane(Sigma-Aldrich, ≧99.8% purity) to form a polymer solution (3-5% wt), andplaced on rollers for at least 12 hours for mixing. After mixing,polymer dense films were prepared by a solution casting method in aglove bag at room temperature to achieve a slow solvent evaporationrate. After solvent was evaporated (usually in 3-4 days), films wereremoved from the casting setting and placed in a vacuum oven at 110° C.for at least 12 hours to remove residual solvent. Once the films wereremoved from the oven, they were cut into small discs with a diameter of2.54 cm. All films had a thickness of approximately 60±10 μm forconsistency.

The polymer films were then pyrolyzed in the exemplary pyrolysisapparatus described above and as illustrated in FIG. 1. A pyrolysistemperature of 550° C. and a two hour soak time was used as atemperature protocol, with the same ramp rates and soak times used byGeiszler and Koros. (V. C. Geiszler and W. J. Koros, Effects ofPolyimide Pyrolysis Conditions on Carbon Molecular Sieve MembraneProperties, Ind. Eng. Chem. Res., 35, 2999 (1996).) Pyrolysis wasperformed on film samples using four different pyrolysis atmospheres: 4ppm O₂/Ar, 8 ppm O₂/Ar, 30 ppm O₂/Ar, and 50 ppm O₂/Ar, each flowedthrough the pyrolysis chamber at a rate of 200 cc (STP)/min. Anadditional film was prepared using an 8 hour thermal soak time at 550°C., and with a pyrolysis atmosphere of 7 ppm O₂/Ar.

After the CMS films were produced by the foregoing process, they wereimmediately loaded into permeation cells. Additionally, a permeationcell was prepared using a non-pyrolyzed 6FDA/BPDA-DAM polyimide film.The films were first masked using impermeable aluminum tape, and only aspecific area was exposed for permeation. Epoxy (Devcon, Danvers, Mass.)was applied at the interface of the tape and the film to furtherminimize any gas leak. This assembly was placed in a double O-ringflange permeation cell. Each cell was placed in a permeation system inwhich a constant-volume variable pressure method was employed, accordingto the methods disclosed by Pye, et al. (D. G. Pye, H. H. Hoehn, and M.Panar, Measurement of Gas Permeability of Polymers I, J. Appl. Polym.Sci., 20, 1921 (1976) and D. G. Pye, H. H. Hoehn, and M. Panar,Measurement of Gas Permeability of Polymers II, J. Appl. Polym. Sci.,20, 287 (1976).) Both upstream and downstream of the permeation systemwere evacuated for at least 12 hours, and a leak rate is measured, whichwas less than 1% of the permeability rate of the slowest gas. Once thesystem was evacuated, the upstream was pressured with a testing gascontaining a testing gas of either CO₂ or CH₄ while the downstream wasmaintained at a vacuum, but isolated from the vacuum pump. Thetemperature of the system was set at 35° C. The pressure rise in astandard volume on the downstream was calculated with time by a dataacquisition software, such as LabView (National Instruments, Austin,Tex.), and CO₂ permeability and CO₂/CH₄ selectivity were calculated. Thesystem was evacuated each time before experiments with different gasesfor at least 12 hours.

Experimental Results:

1. Separation Performance of CMS Membranes Produced from 6FDA-BPDA-DAMPolyimide with Varying Pyrolysis Oxygen Concentrations

FIG. 3 illustrates the results of the CO₂ permeability and CO₂/CH₄selectivity tests on the five CMS films pyrolyzed with varyingconcentrations of oxygen in the pyrolysis atmosphere, as well as the CO₂permeability and CO₂/CH₄ selectivity of a non-pyrolyzed 6FDA/BPDA-DAMpolyimide film. As FIG. 3 shows, each of the pyrolyzed CMS filmsexhibited higher CO₂ permeability and higher CO₂/CH₄ selectivity thanthe non-pyrolyzed precursor film. Additionally, FIG. 3 shows that CO₂permeability decreased with increasing oxygen concentration in thepyrolysis atmosphere. At the same time, CO₂/CH₄ selectivity increasedbetween 4 ppm O₂/Ar and 30 ppm O₂/Ar, but then showed a decrease againwith the 50 ppm O₂/Ar film. Also indicated is the so-called “RobesonUpper bound” for polymeric membranes that gives the theoreticalseparation performance boundary for glassy polymer membranes. (L.Robeson, Journal of Membrane Science, 62 (1991), p 168-185.)

2. Separation Performance of CMS Membranes Produced from 6FDA-BPDA-DAMPolyimide with Varying Soak Time

FIG. 3 also illustrates that thermal soak time does not have asignificant impact on CO₂ permeability and CO₂/CH₄ selectivity forpyrolyzed 6FDA-BPDA-DAM polyimide as compared to pyrolysis atmosphereoxygen concentration. As shown in FIG. 3, the CO₂ permeability andCO₂/CH₄ selectivity for a CMS film prepared using a 2 hour soak time and8 ppm O₂/Ar was nearly identical to, and within the range ofexperimental error, to the values for a CMS film prepared using an 8hour soak time and 7 ppm O₂/Ar.

Example 2 CMS Films from Matrimid® Polyimide

A CMS film is prepared using Matrimid® 5218, a commercially availablepolyimide available from Huntsman Advanced Materials (formerly Vantico,Inc.). Matrimid® 5218 is a thermoplastic polyimide based on aproprietary diamine, 5(6)-amino-1-(4′ aminophenyl)-1,3,-trimethylindane.Homogenous polymeric dense films were prepared using the same procedureas in Example 1. The polymer films were then pyrolyzed in the exemplarypyrolysis apparatus of Example 1. A pyrolysis temperature of 550° C. anda two hour soak time was used as a temperature protocol, with the sameramp rates and soak times used in Example 1. Pyrolysis was performed onfilm samples using six different pyrolysis atmospheres: vacuum, 3 ppmO₂/Ar, 10 ppm O₂/Ar, 30 ppm O₂/Ar, 50 ppm O₂/Ar, and 100 ppm O₂/Ar.Further, CMS films were also prepared to test possible variations basedon other factors. Films were prepared using 30 ppm O₂/Ar oxygenconcentration and two different flow rates; 50 cc (STP)/min and 200cc(STP)/min. Additionally, films were prepared using 30 ppm O₂/Ar oxygenand two different film thicknesses; 4 mil and 2 mil.

Experimental Results:

1. Separation Performance of CMS Membranes Produced from Matrimid®Precursors with Varying Pyrolysis Oxygen Concentrations

FIG. 4 illustrates the results of the CO₂ permeability and CO₂/CH₄selectivity tests on the six CMS films pyrolyzed with varyingconcentrations of oxygen in the pyrolysis atmosphere, as well as the CO₂permeability and CO₂/CH₄ selectivity of a non-pyrolyzed 6FDA/BPDA-DAMpolyimide film. As FIG. 4 shows, each of the pyrolyzed CMS filmsexhibited higher CO₂ permeability than the non-pyrolyzed polymericprecursor film. Additionally, FIG. 4 shows that CO₂ permeability andCO₂/CH₄ selectivity both decreased with increasing oxygen concentrationin the pyrolysis atmosphere. Also indicated is the so-called “Robesonline.”

2. Separation Performance of CMS Membranes Produced from Matrimid®Polymeric Precursors with Varying Soak Time

FIG. 5 illustrates the results of the CO₂ permeability and CO₂/CH₄selectivity tests on the two CMS films pyrolyzed with varying thermalsoak times of 2 hours and 8 hours. As FIG. 5 shows, both permeabilityand selectivity results show that there is very little change in theseparation performance for these membranes with various thermal soaktimes. These results show that total amount of oxygen exposure has verylittle impact on performance. Additionally, these results show that fortemperatures of about 550° C., thermal soak times beyond two hours alsohave very little impact on performance. Therefore, commercial processes,which are often larger and require longer cool down times thanlaboratory-scale processes, will not be impacted by small changes inthermal profile.

3. Separation Performance of CMS Membranes Produced from Matrimid®Precursors with Varying Pyrolysis Atmosphere Flow Rate

FIG. 6 shows the results of the CO₂ permeability and CO₂/CH₄ selectivitytests on the two CMS films pyrolyzed with varying pyrolysis atmosphereflow rates of 50 cc (STP)/min and 200 cc (STP)/min at 30 ppm O₂/Ar. AsFIG. 6 shows, there is little change in gas separation performance basedon pyrolysis atmosphere flow rate, even though a greater flow rate meansthat the total amount of oxygen available was greater. The below table,which provides the results of total O₂ availability and total O₂consumption for each of the two flow rates, also shows that flow ratehas very little impact on the amount of oxygen consumed duringpyrolysis.

Inert flowrate during Total O₂ available Total O₂ consumed pyrolysis (cc(STP)/min) (cc (STP)/g) (cc (STP)/g) 50 67.3 45.4 200 154.2 50.0

4. Separation Performance of CMS Membranes Produced from Matrimid®Polymeric Precursors with Varying Precursor Polymer Thickness

FIG. 7 illustrates the results of the CO₂ permeability and CO₂/CH₄selectivity tests on the two CMS films having varying thicknesses of 4mil and 2 mil. As FIG. 7 shows, both permeability and selectivityresults show that there is very little change in the separationperformance for these membranes with different thicknesses. Theseresults show that the oxygen reaction in the pores of the carbonmolecular sieve is not limited by internal mass transfer. In addition,these results indicate that the membranes are symmetric, and that oxygenincorporation on the membranes is uniform throughout. This shows thatthe present technology is not limited to any particular membranegeometry or dimensions.

The comparative tests using varying thermal soak rate, varyingatmosphere flow rate, and varying membrane thickness indicate that thetransport mechanism is most likely limited by chemical reaction, whichis controlled by oxygen concentration. It is believed, therefore, thatonly concentration and temperature are important in theequilibrium-controlled case.

We claim:
 1. A process of separating acid gas components from a naturalgas stream comprising: providing an asymmetric carbon molecular sievehollow fiber module comprising a sealable enclosure, said enclosurehaving a plurality of membranes contained therein, at least one of saidmembranes produced according to the process comprising the steps of a.providing an asymmetric hollow polymer fiber precursor b. heating saidprecursor in a chamber to at least a temperature at which pyrolysisbyproducts are evolved; and, c. flowing an inert gas through saidchamber, said inert gas containing less than about 40 ppm of oxygen; aninlet for introducing a feed stream comprising natural gas; an outletfor permitting egress of a permeate gas stream; and, another outlet forpermitting egress of a retentate gas stream; and introducing saidnatural gas stream into said module at a pressure of at least about 20bar.
 2. The process of claim 1, wherein said natural gas stream containsat least about 3% by volume of an acid gas component.
 3. A process forreducing the concentration of acid gases in a natural gas streamcomprising: (a) providing a carbon membrane produced by a processincluding the steps of: i. providing a membrane formed of asymmetrichollow polymer fibers; ii. heating said membrane in a chamber to atleast a temperature at which pyrolysis byproducts are evolved; and, iii.flowing an inert gas through said chamber, said inert gas containingless than about 40 ppm of oxygen, (b) flowing said natural gas streamthrough said membrane to produce a retentate stream having a reducedconcentration of acid gases.
 4. The process of claim 3, wherein saidinert gas contains less than about 10 ppm of oxygen.
 5. A process foroptimizing the CO₂/CH₄ selectivity of a carbon membrane comprisingforming said carbon membrane by: a. providing a polymer precursor in theform of asymmetric hollow polymer fibers; b. heating said precursor in achamber to at least a temperature at which pyrolysis byproducts areevolved; and, c. flowing an inert gas through said chamber, said inertgas containing less than about 10 ppm of oxygen.
 6. The process of claim5, wherein said precursor is in the form of an asymmetric hollow polymerfiber.
 7. A carbon membrane produced by the process of claim
 5. 8. Aprocess for making a carbon membrane having a predetermined degree ofCO₂ permeability comprising: a. providing a polymer precursor b. heatingsaid precursor in a chamber to at least a temperature at which pyrolysisbyproducts are evolved; and, c. flowing a gas through said chamber, theconcentration of oxygen in said gas being selected to produce a carbonmembrane having said predetermined degree of CO₂ permeability.
 9. Theprocess of claim 8, said oxygen concentration being selected to increasethe permeability of said carbon membrane.
 10. The process of claim 8,wherein said oxygen concentration is maintained between 1 ppm and 30ppm.
 11. The process of claim 8 wherein the heating temperature is alsoselected to provide a predetermined degree of CO₂ permeability.
 12. Theprocess of claim 11 wherein the heating temperature is between 450° C.and 600° C.
 13. The process of claim 11 wherein the heating temperatureis between 600° C. and 1000° C.
 14. A process according to claim 8,further comprising a. providing a second polymer precursor b. heatingsaid second polymer precursor in a chamber to at least a temperature atwhich pyrolysis byproducts are evolved; and, c. flowing a gas throughsaid chamber, the concentration of oxygen in said gas being selected toproduce a second carbon membrane having a predetermined degree of CO₂permeability; whereby a first carbon membrane having a firstpredetermined degree of CO₂ permeability is produced and a second carbonmembrane having a second predetermined degree of CO₂ permeability isproduced, said first and second predetermined degrees of CO₂permeability differing by at least about 10 Barrer.
 15. A process formaking a carbon membrane having a predetermined degree of CO₂/CH₄selectivity comprising: a. providing a polymer precursor b. heating saidprecursor in a chamber to at least a temperature at which pyrolysisbyproducts are evolved; and, c. flowing a gas through said chamber, theconcentration of oxygen in said gas being selected to produce a carbonmembrane having said predetermined degree of CO₂/CH₄ selectivity. 16.The process of claim 15, said oxygen concentration being selected toincrease the CO₂/CH₄ selectivity of said carbon membrane.
 17. Theprocess of claim 15, wherein said oxygen concentration is maintainedbetween 1 ppm and 30 ppm.
 18. The process of claim 15, wherein saidoxygen concentration is maintained at between 30 ppm and 50 ppm.
 19. Theprocess of claim 15 wherein the heating temperature is also selected toprovide a predetermined degree of CO₂/CH₄ selectivity.
 20. The processof claim 19 wherein the heating temperature is between 450° C. and 600°C.
 21. The process of claim 19 wherein the heating temperature isbetween 600° C. and 1000° C.
 22. A process according to claim 15,further comprising a. providing a second polymer precursor b. heatingsaid second polymer precursor in a chamber to at least a temperature atwhich pyrolysis byproducts are evolved; and, c. flowing a gas throughsaid chamber, the concentration of oxygen in said gas being selected toproduce a second carbon membrane having a predetermined degree ofCO₂/CH₄ selectivity; whereby a first carbon membrane having a firstpredetermined degree of CO₂/CH₄ selectivity is produced and a secondcarbon membrane having a second predetermined degree of CO₂/CH₄selectivity is produced, said first and second predetermined degrees ofCO₂/CH₄ selectivity differing by at least about
 10. 23. A gas separationapparatus, comprising one carbon membrane module produced by pyrolyzinga polymer precursor in an atmosphere having a first oxygenconcentration, and another carbon membrane module produced by pyrolyzinga polymer precursor in an atmosphere having a second oxygenconcentration, said first and second oxygen concentrations differing byat least 2 ppm oxygen to inert gas.
 24. The apparatus of claim 23,wherein said first and second oxygen concentrations differ by at least10 ppm oxygen to inert gas.
 25. The apparatus of claim 23, comprisingone carbon membrane having a first CO₂ permeability, and another carbonmembrane having a second CO₂ permeability, said first and second CO₂permeabilities differing by at least about 10 Barrer.
 26. The apparatusof claim 23 comprising one carbon membrane having a first CO₂/CH₄selectivity, and another carbon membrane having a second CO₂/CH₄selectivity, said first and second first CO₂/CH₄ selectivities differingby at least about
 10. 27. A process for making at least two carbonmembranes having different predetermined degrees of CO₂ permeabilitycomprising the steps of: a. providing a first polymer precursor; b.heating said first precursor in a first chamber to at least atemperature at which pyrolysis byproducts are evolved; c. flowing afirst gas through said first chamber during said first heating step, theconcentration of oxygen in said first gas being selected to produce afirst carbon membrane having a first predetermined degree of CO₂permeability; d. providing a second polymer precursor; e. heating saidsecond precursor in a second chamber to at least a temperature at whichpyrolysis byproducts are evolved; and f. flowing a second gas throughsaid second chamber during said second heating step, the concentrationof oxygen in said second gas being selected to produce a second carbonmembrane having a second predetermined degree of CO₂ permeability,wherein the concentration of oxygen in said first gas and said secondgas differ by at least 2 ppm.
 28. The first and second carbon membranesmade by the process of claim
 27. 29. A process for making at least twocarbon membranes having different predetermined degrees of CO₂/CH₄selectivity comprising the steps of: a. providing a first polymerprecursor; b. heating said first precursor in a first chamber to atleast a temperature at which pyrolysis byproducts are evolved; c.flowing a first gas through said first chamber during said first heatingstep, the concentration of oxygen in said first gas being selected toproduce a first carbon membrane having a first predetermined degree ofCO₂/CH₄ selectivity; d. providing a second polymer precursor; e. heatingsaid second precursor in a second chamber to at least a temperature atwhich pyrolysis byproducts are evolved; and f. flowing a second gasthrough said second chamber during said second heating step, theconcentration of oxygen in said second gas being selected to produce asecond carbon membrane having a second predetermined degree of CO₂/CH₄selectivity, wherein the concentration of oxygen in said first gas andsaid second gas differ by at least 2 ppm.
 30. The first and secondcarbon membranes made by the process of claim
 29. 31. An asymmetriccarbon molecular sieve hollow fiber module comprising a sealableenclosure, said enclosure having; a plurality of membranes containedtherein, at least one of said membranes produced according to theprocess of claim 8; an inlet for introducing a feed stream comprisingnatural gas; an outlet for permitting egress of a permeate gas stream;and, another outlet for permitting egress of a retentate gas stream. 32.An asymmetric carbon molecular sieve hollow fiber module comprising asealable enclosure, said enclosure having; a plurality of membranescontained therein, at least one of said membranes produced according tothe process of claim 15; an inlet for introducing a feed streamcomprising natural gas; an outlet for permitting egress of a permeategas stream; and, another outlet for permitting egress of a retentate gasstream.
 33. The process of claim 8, wherein said oxygen concentration ismaintained between 8 ppm and 40 ppm.
 34. The process of claim 15,wherein said oxygen concentration is maintained between 8 ppm and 40ppm.
 35. The process of claim 8, wherein said oxygen concentration ismaintained between 3 ppm and 100 ppm.
 36. The process of claim 15,wherein said oxygen concentration is maintained between 3 ppm and 100ppm.